1. Field of the Invention
This invention relates to a method of generating a transmission signal by processing which includes digital quadrature modulation. Also, this invention relates to an apparatus for generating a transmission signal by processing which includes digital quadrature modulation.
2. Description of the Related Art
Japanese patent application publication number 8-102766 discloses digital quadrature modulators in which an I-channel (in-phase channel) carrier is regarded as a repetitive data sequence of “1”→“0”→“−1”→“0”, and a Q-channel (quadrature channel) carrier is regarded as a repetitive data sequence of “0”→“1”→“0”→“−1”. A digital I-channel information signal is sequentially multiplied by the I-channel carrier data sequence, while a digital Q-channel information signal is sequentially multiplied by the Q-channel carrier data sequence. A signal generated by the multiplication in the I-channel and a signal generated by the multiplication in the Q channel are multiplexed into a digital-quadrature-modulation result signal.
In the case where samples of the digital I-channel information signal and samples of the digital Q-channel information signal are synchronized with each other, the 90-degree (π/2) phase difference between the I-channel and Q-channel carrier data sequences causes a timing phase difference between the I-channel components and the Q-channel components of the digital-quadrature-modulation result signal. Such a timing phase difference adversely affects signal transmission.
Japanese application 8-102766 discloses that digital filters are provided respectively in I-channel and Q-channel signal flow paths before a stage for multiplexing the digital I-channel and Q-channel information signals by the I-channel and Q-channel carrier data sequences. The I-channel and Q-channel digital filters are designed to provide different signal phases to compensate for the timing phase difference between the I-channel components and the Q-channel components of the digital-quadrature-modulation result signal.
The I-channel and Q-channel digital filters in Japanese application 8-102766 are required to implement accurate operation. In addition, the I-channel and Q-channel digital filters have complicated structures, and are hence expensive.